The present invention relates to an image reading device which is provided with an image sensor including a number of photoelectric conversion elements arranged in a line to read an image on a document.
Conventionally, an image reading device for use in digital image forming apparatus, such as facsimile machine or copying machine, is provided with an image sensor including an array of a number of photoelectric conversion elements such as CCD (Charge Coupled Device) arranged in a line. The image reading device reads an image on a document by scanning photoelectric conversion elements from one end of the array to the other end at a predetermined interval (hereinafter referred to as "main scanning") while moving the image sensor or the document in relative to each other in a direction perpendicularly intersecting the main scanning direction (hereinafter referred to as "sub-scanning").
In such image reading devices, to read an image on a document whose size is larger in a main scanning direction, the length of the photoelectric conversion array is required to be made longer to cover the larger width of the document. In other words, a greater number of photoelectric conversion elements are required to be arranged in a line. However, this arrangement takes a greater deal of time for one main scanning, consequently increasing the image reading time.
To solve this problem, conventional image reading devices have adopted a so-called multi-tap arrangement in which photoelectric conversion elements of an image sensor are divided into a plurality of groups and each group is provided with a scan controller. This arrangement can prevent the reading time from increasing.
For example, photoelectric conversion elements are divided into three groups, i.e., first, second, and third groups. The photoelectric conversion elements of the first, second, and third groups are scanned in synchronism with one another from the first element to the n-th element. Accordingly, the image reading time can be reduced into substantially one third of that of a non-divided image sensor.
However, in the case of reading an image on a document whose size in a main scanning direction is identical to the length of one photoelectric conversion group, for example, in the case of reading an A4-sized document in its widthwise direction by an image sensor whose one photoelectric conversion group has a length identical to the shorter side of the A4-sized sheet, the above-mentioned advantage of the multi-tap arrangement cannot be taken.
Accordingly, it might be considered to further divide one photoelectric conversion group into smaller groups to enable a smaller sized document to be read by a plurality of smaller groups. It will be certain that the greater the number of photoelectric conversion groups increases, the shorter the reading time becomes. However, the number of scanning controllers inevitably increases and the construction of an image reading device consequently becomes more complicated.